AMPEROMETRIC BIOSENSORS BASED ON IMMOBILIZED ENZYMES AND CHEMICALLY MODIFIED ELECTRODES

Amperometric biosensors based on two different reaction mechanisms are presented. Common to both types is the combination of a selective catalytic reaction that can be followed amperometrically at 0 mV vs SCE and below. One type is based on the chemical modification of carbon pastes with a dehydrogenase, the necessary cofactor NAD’, and a redox mediator. In the presence of the enzyme substrate NADH will be produced. The high overvoltage for the electrochemical oxidation of the NADH is decreased by the addition of the redox mediator to the paste. The redox mediators used are phenoxazine derivatives making the electrocatalytic oxidation of NADH possible at 0 mV vs SCE and below. A glucose sensor based on glucose dehydrogenase is described. Another type is based on the co-immobilization of a hydrogen peroxide producing oxidase with horse radish peroxidase on the surface of heat-treated graphite. The detection is based on an apparent direct electron transfer from the electrode to the immobilized peroxidase starting at +600 mV and reaching a maximum at about O mV vs SCE. The co-immobilized enzyme layer is stabilised by the addition of bovine serum albumin and glutaraldehyde to the reaction mixture. A glucose sensor based on glucose oxidase is presented